This application relates to and claims priority from Japanese Patent Application Nos. 2001-69184, filed on Mar. 12, 2001, and 2001-215657, filed on Jul. 16, 2001, the contents of which are hereby incorporated by reference.
This invention relates to a method of manufacturing a semiconductor substrate, such as an SOI substrate, and a method of measuring film thickness of an active layer (semiconductor layer) on an SOI substrate comprising an oxide film sandwiched between two silicon substrates.
A silicon wafer is mirror-polished after going through a slicing step, lapping step, etching step, and a polishing step. The mirror-polishing step is specifically accomplished by pasting a multitude of wafers on a multitude of rotating polishing blocks with, for example, wax and pressing wafers on the surface of a rotating polishing plate to which a polishing cloth is attached.
In recent years, there has been an increased need to make sure that surfaces of silicon wafers are flat and parallel to each other. Silicon wafer thickness must be controlled closely to meet this need. When forming an SOI substrate by pasting together two silicon substrates, film thickness control during polishing becomes a critical issue for obtaining an active layer of the specified thickness.
In a conventional process for polishing an SOI substrate, steps for rough polishing are performed for a specified amount of time, film thickness of the active layer is measured by taking the SOI substrate out of a polishing system, and steps for fine polishing and film thickness measurement are repeated until the active layer attains the specified film thickness. Because the SOI substrate would become unusable with excess polishing, repeated polishing and film thickness measurement steps are needed. Taking the substrate out of the polishing system for film thickness measurement is required for each iteration. This results in many steps and more polishing time.
To address this problem, various methods of measuring the film thickness during polishing for a monitoring purpose have been explored. Out of those, the most promising technology is a method of measuring the film thickness based on optical interference (See Japanese unexamined patent publication (JP-A) No. H8-216016).
A study by the inventors, however, has revealed that that a time lag, which results from collecting the interference data for a wavelength spectrum during an optical interference process for film thickness measurement, makes it difficult to obtain an accurate film thickness measurement, because the film thickness changes during the time lag.
In addition, in an SOI substrate structure comprising two silicon substrates sandwiching an oxide film, optical interference is weakened by light beams reflecting from both sides of the oxide film, because the oxide film is sandwiched between materials of the same optical characteristics. This problem results in xe2x80x9cnodesxe2x80x9d with weakened optical interference at specific oxide film thicknesses. It is difficult to take measurements at wavelengths corresponding to the nodes.
FIGS. 23A and 23B show relationships between wavelength and reflectance, obtained by simulation, when film thicknesses are 5 xcexcm for the active layer and 1 xcexcm and 2 xcexcm, respectively, for the oxide film. As FIG. 23A shows, an optical-interference wave pattern shows a maximum around a wavelength of 830 nm. However, when there is a node near this wavelength, the value at this wavelength becomes a minimum, as FIG. 23B shows. In other instances, the wave form might not show a peak or a maximum value. An accurate film thickness measurement is impossible when there are no peaks where they are supposed to be.
An object of this invention is to address the issues discussed above, take into account the time lag associated with the collection of interference data, and facilitate accurate film-thickness measurements. Another objective of this invention is to make accurate film-thickness measurement possible by taking into account the nodes at which optical interference is weakened.
The invention is basically a method of measuring the film thickness of an active layer on an SOI substrate. The SOI substrate includes the active layer, which is a semiconductor layer, and a supporting substrate. The active layer and the supporting substrate sandwich an oxide film. The method includes radiating light on the SOI substrate. A range of analysis wavelengths is selected to eliminate wavelengths at which optical interference is weakened by light beams reflected from both surfaces of the oxide film. The method includes breaking down light that is reflected from the SOI substrate into a spectrum by wavelength. Complete optical interference data is collected by wavelength of the reflected light. Finally, the film thickness of the active layer is calculated based on the interference data in the selected wavelength range.
In another aspect of the invention, the range of analysis wavelengths includes wavelengths xcex that satisfy the following inequality:             2      ·              n                  o          ⁢                      xe2x80x83                    ⁢          x                    ·              d                  o          ⁢                      xe2x80x83                    ⁢          x                            m      -      0.98         greater than   λ   greater than             2      ·              n                  o          ⁢                      xe2x80x83                    ⁢          x                    ·              d                  o          ⁢                      xe2x80x83                    ⁢          x                            m      -      0.02      
where nox is an refractive index for the oxide film, dox is the thickness of the oxide film, and m is an arbitrary positive integer.
In another aspect of the invention the film thickness d of the active layer is calculated with the following equation:   d  =                              λ          1                ·                  λ          2                                                  λ            2                    ·                      n            ⁢                          (                              λ                1                            )                                      -                              λ            1                    ·                      n            ⁢                          (                              λ                2                            )                                            ·                  (                  X          +                      s            1                    -                      s            2                          )            2      
where xcex1 and xcex2 are wavelengths corresponding to different peak values, respectively, n(xcex1) is the film""s refractive index at wavelength xcex1, n(xcex2) is the film""s refractive index at wavelength xcex2, X is the wave number between the two wavelengths, and s1 and s2 are phase shift amounts between a single layer film structure and a double layer film structure at the wavelengths corresponding to the peak values.
Other features, objects and advantages of the invention will become apparent from the following description of preferred embodiments made in reference to the accompanying drawings, which form a part of the specification.